Relations between Britain and Brazil have struck a new low after a careless slip by the prime minister, Tony Blair, during a major foreign policy speech in Washington last week.

The timing of the diplomatic spat is unfortunate for Mr Blair, who has been courting Brazil's president, Lula da Silva, in an effort to break the deadlock in international trade talks.

The Brazilian foreign ministry confirmed to the Guardian yesterday that its foreign minister, Celso Amorim, called the British ambassador in Rio, Peter Collecott, into his office on Tuesday to seek an explanation for Mr Blair's apparent gaffe.

The row began after Mr Blair spoke in Washington about the need to reform the United Nations by expanding permanent membership of the security council - which represents the victorious allies after the second world war - from five to include leading industrial powers and representatives from the developing world.

The British government has long championed the membership of Brazil, even though Argentina has a strong claim as a representative from Latin America.

But when Mr Blair made his speech, he said: "A security council which has France as a permanent member but not Germany, Britain but not Japan, China but not India, to say nothing of the absence of proper representation from Latin America or Africa, cannot be legitimate in the modern world."

The Brazilians immediately picked up on the sudden absence of their country.

Mr Blair scribbled down the speech, the third in a series of wide-ranging foreign policy speeches, on the flight from London to Washington. According to a British source, he did not seek to change British policy with regard to Brazil but was simply setting out his overall argument about a need for UN reform.

A Foreign Office spokesman said yesterday: "We remain supportive of Brazil's candidature, along with Germany, Japan and India, for permanent seats on an enlarged security council. The speech was not intended to set out our policy in detail but was an attempt to inject momentum into the debate. We remain solidly supportive of Brazil's bid for a permanent seat."

Mr Blair, along with the American president, George Bush, and the German chancellor, Angela Merkel, has been trying to secure the support of five leading developing countries, including Brazil, for a special trade conference to be held around the time of next month's G8 summit in St Petersburg.